Chapter 16 Parents to offsspring 2

Chapter 16 – Transmission of Genetic Information from Parents to
Offspring II: Epigenetics, Linkage, and Extranuclear Inheritance
Chapter Outline
1. Overview of epigenetics
2. Epigenetics: genomic
imprinting
3. Epigenetics: X-chromosome
inactivation
4. Epigenetics: effects of
environmental agents
5. Extranuclear inheritance:
organelle genomes
6. Linkage of genes on the
same chromosome

16.1 Overview of Epigenetics
Section 16.1 Learning
Outcomes
1. Define epigenetics and
epigenetic inheritance
2. Outline the types of
molecular changes that
underlie epigenetic effects
on gene expression

16.1 Overview of Epigenetics
• Female honeybees are of two types: queen bees and worker bees
• The differences between queens and workers are not due to
differences in alleles, rather the difference in development is due
to epigenetic modifications related to differences in their diet
• Although many genes
follow Mendelian
inheritance patterns,
some genes follow
other patterns

16.1 Overview of Epigenetics
• Epigenetics is the study of mechanisms that lead to changes in
gene expression
• that can be passed from cell to cell
• that are reversible
• that do not involve a change in the sequence of DNA
• Some epigenetic changes repress transcription while others
activate transcription
• Some epigenetic changes are relatively permanent during an
individual’s life while others may be reversible
• Epigenetic inheritance is the passage of an epigenetic change from
parent to offspring via sperm or egg cells

16.1 Overview of Epigenetics
• Common types of molecular changes that underlie epigenetic effects
on gene expression are DNA methylation, chromatin remodeling,
covalent histone modification, and localization of histone variants

16.2 Epigenetics: Genomic Imprinting
Section 16.2 Learning
Outcomes
1. Explain how the Igf2 gene
exemplifies the
phenomenon of genomic
imprinting
2. Explain the molecular basis
of genomic imprinting
3. Predict the outcome of
crosses for imprinted genes

16.2 Epigenetics: Genomic Imprinting
• Genomic imprinting refers to a segment of DNA being “marked”;
the mark is retained and recognized throughout the life of the
organism
• A gene can be marked by females during egg formation or by
males during sperm production, but not both
• The marking process involves epigenetic modifications and
affects whether or not the gene is expressed
• The offspring expresses either the maternal allele or the paternal
allele, but not both
• Imprinted genes do not follow a Mendelian pattern of
inheritance
• Several hundred to over a thousand human genes are thought to
be imprinted

16.2 Epigenetics: Genomic Imprinting
For Imprinted Genes, the Gene from Only One Parent
Is Expressed

16.2 Epigenetics: Genomic Imprinting
For Imprinted Genes, the Gene from Only One Parent
Is Expressed
• If female parent is homozygous for the mutant allele  all normal
• If male parent is homozygous for mutant allele  all dwarf
offspring
• The normal size and dwarf offspring all have the same genotype
(Igf2 Igf2-) but have different phenotypes
• Igf2 is imprinted such that only the paternal allele is expressed

16.2 Epigenetics: Genomic Imprinting
DNA Methylation Affects the
Transcription of an Imprinted Gene
• DNA methylation is the marking process
that occurs during the imprinting of
certain genes, including Igf2
• For most genes, methylation silences
gene expression; for some it may
enhance gene expression
• Each time a somatic cell divides, the
methylation state is retained
• The methylation state can be altered
when individuals make gametes
• The reactions that add or remove methyl
groups are catalyzed by enzymes

16.3 Epigenetics: X-Chromosome Inactivation
Section 16.3 Learning
Outcomes
1. Explain how X-chromosome
inactivation may affect the
phenotype of female
mammals
2. Describe the process of X-
chromosome inactivation at
the cellular level
3. Explain how calico cats
exemplify the phenomenon
of X-chromosome
inactivation
4. Predict the outcome of
crosses for X-linked genes

16.3 Epigenetics: X-Chromosome Inactivation
• Female mammals carry two X chromosomes (XX) whereas male
mammals carry one X chromosome (XY)
• During embryonic development in female mammals, one of
the X chromosomes undergoes an epigenetic change called
X-chromosome inactivation (XCI)
• The inactivated X chromosome (Barr body) becomes highly
compacted, which silences the genes that it carries

16.3 Epigenetics: X-chromosome Inactivation
In Female Mammals, One X Chromosome Is Inactivated
in Each Somatic Cell
• The patchy pattern of calico cats is due to permanent inactivation
of one X chromosome in each skin cell
• Coat color is determined by
an X-linked gene
• Orange allele, XO
• Black allele, XB
• Heterozygous XOXB
female will be calico

16.3 Epigenetics: X-chromosome Inactivation
In Female Mammals, One X Chromosome Is Inactivated
in Each Somatic Cell
• In early development, one of the
two X chromosomes is randomly
inactivated in each somatic cell,
including the cells that give rise
to the hair-producing skin cells
• The pattern of XCI is maintained
during subsequent cell divisions
• Female mammals can be
described as mosaics because
they are composed of two types
of cells

16.3 Epigenetics: X-chromosome Inactivation
In Female Mammals, One X Chromosome Is Inactivated
in Each Somatic Cell
• It is proposed that XCI achieves dosage compensation, equalization
of levels of expression of X-linked genes in male and female cells
• Cells of humans and other mammals have the ability to count X
chromosomes and allow only one X chromosome to be active
• Extra X chromosomes are converted to Barr bodies

16.3 Epigenetics: X-chromosome Inactivation
The X Chromosome Has an X Inactivation Center That
Controls Compaction into a Barr Body
• A short region of the X chromosome called the X inactivation
center (Xic) plays a critical role in XCI
• The expression of a specific gene, Xist (X inactive specific transcript)
is required for the compaction of the X chromosome into a Barr
body
• The Xist gene product is a long RNA molecule that does not encode
a protein; instead, the Xist RNA coats one of the two X
chromosomes during the process of XCI
• After coating, proteins associate with the Xist RNA and cause
epigenetic changes that promote the compaction of the
chromosome
• This chromosome is maintained as a Barr body for an
individual’s life

16.4 Epigenetics: Effects of Environmental Agents
Section 16.4 Learning
Outcomes
1. Explain how chemicals in
the diet may affect an
individual's phenotype
2. Explain how the Agouti
gene in mice exemplifies
the phenomenon of
environmental agents
causing epigenetic changes
3. Give an example of an
environmental agent that is
known to cause epigenetic
changes and is associated
with cancer

16.4 Epigenetics: Effects of Environmental Agents
Chemicals in an Individual’s Diet May Cause Epigenetic
Changes That Affect Phenotype
• Studies of the Agouti gene in mice have demonstrated how
chemicals in the diet can promote epigenetic changes
• This gene encodes the Agouti signaling peptide that controls the
deposition of yellow pigment in developing hairs
• Several mutations have been identified; Avy is a gain of function
mutation due to the insertion of a new (and very active)
promoter next to the normal promoter
• Overexpression of Agouti causes mice
to be yellow, however there is wide
variation in phenotype of mice carrying
the Avy allele

16.4 Epigenetics: Effects of Environmental Agents
Chemicals in an Individual’s Diet May Cause Epigenetic
Changes That Affect Phenotype
• The new promoter in Avy mice is very sensitive to epigenetic
changes; coat colors correlate with the degree of methylation at
the new promoter
• Nutrients known to influence DNA methylation affect coat color in
Avy mice; offspring of females fed a supplemented diet tended to
have darker coats

16.4 Epigenetics: Effects of Environmental Agents
Environmental Agents May Cause Epigenetic Changes
That Are Associated with Human Diseases Like Cancer
• Researchers have identified many examples in which epigenetic
changes are associated with a particular disease (Alzheimer,
diabetes, multiple sclerosis, asthma, cardiovascular diseases)
• The role of epigenetics in cancer has been most studied; several
environmental factors have been associated with specific cancers

16.5 Extranuclear Inheritance: Organelle Genomes
Section 16.5 Learning
Outcomes
1. Describe the general
features of mitochondrial
and chloroplast genomes
2. Explain why chloroplast and
mitochondrial genes usually
exhibit maternal inheritance
3. Give an example of a
human disease associated
with mutations in
mitochondrial genes
4. Predict the outcome of
crosses for mitochondrial or
chloroplast genes

16.5 Extranuclear Inheritance: Organelle Genomes
• The transmission of genes
located outside the nucleus is
called extranuclear inheritance
• Mitochondria and chloroplasts
contain their own genome
• The endosymbiosis theory
describes the origin of these
semiautonomous organelles

16.5 Extranuclear Inheritance: Organelle Genomes
Chloroplast and Mitochondrial Genomes Are Small but
Contain Genes That Encode Important Proteins
• Mitochondrial and chloroplast
genomes are composed of a single,
circular DNA molecule
• Typically, the mammalian
mitochondrial genome has 37 genes
• 24 encode tRNAs and rRNA
needed for translation inside
the mitochondrion
• 13 encode proteins for oxidative
phosphorylation
• Chloroplast genomes of flowering
plants contain 100-200 genes
• Many encode proteins vital to
photosynthesis

16.5 Extranuclear Inheritance: Organelle Genomes
Chloroplast Genomes Are Often Maternally Inherited
• Leaf pigment in the four-o’clock plant does
not obey Mendel’s law of segregation
• Leaf pigmentation of offspring depends
solely on the pigmentation of the maternal
plant; this phenomenon is called maternal
inheritance
• The white phenotype is caused by a
mutation in a gene within the chloroplast
genome that prevents the synthesis of
most chlorophyll
• The maternal inheritance pattern occurs
because the chloroplasts are only
transmitted through the cytoplasm of the
egg

16.5 Extranuclear Inheritance: Organelle Genomes
Chloroplast Genomes Are Often Maternally Inherited
• Chloroplasts develop from proplastids
• An egg cell contains several proplastids so an offspring from a
variegated maternal plant can be green, white or variegated
• In seed-bearing plants, maternal
inheritance of chloroplasts is the
most common transmission pattern
• Some species exhibit biparental
inheritance (both pollen and egg
contribute chloroplasts) and others
exhibit paternal inheritance (only
pollen contributes chloroplasts)

16.5 Extranuclear Inheritance: Organelle Genomes
Mitochondrial Genomes Are Maternally Inherited in
Humans and Most Other Species
• Maternal inheritance is the most common pattern of mitochondrial
transmission in eukaryotic species
• Some species do exhibit biparental or paternal inheritance
• Mutations in human
mitochondrial genes can
cause a variety of rare
diseases; usually affect
organs and cells that
require high levels of ATP

16.6 Linkage of Genes on the Same Chromosome
Section 16.6 Learning
Outcomes
1. Explain how linkage violates
the law of independent
assortment
2. Explain how experimental
crosses can demonstrate
linkage
3. Predict the outcome of
crosses for linked genes

16.6 Linkage of Genes on the Same Chromosome
• When two genes are close together on the same chromosome,
they tend to be transmitted as a unit, a phenomenon called linkage
• A typical chromosome contains hundreds to a few thousand genes
• A group of genes that usually stay together during meiosis is called
a linkage group
• In a two-factor
cross, linked genes
do not follow the
law of independent
assortment

16.6 Linkage of Genes on the Same Chromosome
Bateson and Punnett’s Crosses of Sweet Peas Showed
That Genes Do Not Always Assort Independently
• Bateson and Punnett crossed sweet peas for flower color and pollen
shape; the phenotypes of the F2 generation were unexpected

16.6 Linkage of Genes on the Same Chromosome
Bateson and Punnett’s Crosses of Sweet Peas Showed
That Genes Do Not Always Assort Independently

16.6 Linkage of Genes on the Same Chromosome
Linkage and Crossing Over Produce Nonrecombinant
and Recombinant Types
• Thomas Hunt Morgan obtained similar results in fruit flies; many
more F2 offspring displayed the parental combination of traits
• Morgan proposed 3 ideas to explain his data:
1. When different genes are located on the same chromosome,
the traits that are determined by those genes are more likely
to be inherited together
2. Due to crossing over during meiosis, homologous
chromosomes can exchange pieces of chromosomes and
create new combinations of alleles
3. Frequency of crossing over depends on the distance between
two genes; crossovers are much more likely to occur between
two genes that are farther apart in the chromosome compared
to two genes that are closer together

16.6 Linkage of Genes on the Same Chromosome
Linkage and Crossing Over Produce Nonrecombinant
and Recombinant Types
• In the P generation, b+b+ c+c+ (grey body, straight wings) were
crossed with bb cc (black body, curved wings)
• As expected, all F1 had gray bodies and straight wings (dominant
traits)
• F1 were mated with a fly that was homozygous recessive for
both traits in a test cross

16.6 Linkage of Genes on the Same Chromosome
Linkage and Crossing Over Produce Nonrecombinant
and Recombinant Types
• If the genes for body color and wing shape are on different
chromosomes and assort independently, the F2 offspring will have
four possible phenotypes in a 1:1:1:1 ratio
• Instead, the two most
abundant phenotypes
are those with the
combinations of
characteristics from the
P generation: gray
bodies with straight
wings or black bodies
and curved wings

16.6 Linkage of Genes on the Same Chromosome
Linkage and Crossing Over Produce Nonrecombinant
and Recombinant Types
• Nonrecombinants have trait combinations like those of the
parents while a smaller number of recombinants have different
combinations of traits from the parental generation
• Each recombinant individual has a chromosome that is the product
of a crossover

Chapter 16 Summary
16.1 Overview of epigenetics
• Epigenetic mechanisms do not involve changes in the sequence
of DNA but do cause changes in gene expression
16.2 Epigenetics: genomic imprinting
• For imprinted genes, the gene from only one parent is
expressed (ex: Igf2)
• DNA methylation affects the transcription of an imprinted gene
16.3 Epigenetics: X-chromosome inactivation
• In female mammals, one X chromosome is inactivated in each
somatic cell (ex: calico cat)
• The X chromosome has an X inactivation center that controls
compaction into a Barr body

Chapter 16 Summary
16.4 Epigenetics: effects of environmental agents
• Chemicals in an individual’s diet may cause epigenetic changes
that affect phenotype (ex: Agouti gene in mice)
• Environmental agents may cause epigenetic changes that are
associated with human diseases, such as cancer
16.5 Extranuclear inheritance: organelle genomes
• Chloroplast and mitochondrial genomes are relatively small but
contain genes that encode important proteins
• Chloroplast genomes are often maternally inherited
• Mitochondrial genomes are maternally inherited in humans and
most other species
16.6 Linkage of genes on the same chromosome
• Crosses of sweet peas and flies showed that genes do not always
assort independently
• Linkage and crossing over produce nonrecombinant and
recombinant types